This goes contrary to what I was taught : use singular form. When I model my databases I think in those terms : employee works for 1 to n company, company has 0 to n employee. Since it's a "many to many" relationship you need a third table (entity really) which you could call "working_with". The columns are id_employee and id_company.
This is the basis of the Merise method, but I don't think it's available in English. It makes modeling databases really easy.